The present invention relates to secondary batteries having high specific capacities and particularly to anodes for secondary batteries comprising a host material such as a carbonaceous material capable of absorbing and desorbing lithium in an electrochemical system and lithium metal dispersed in the host material.
Lithium and lithium-ion secondary or rechargeable batteries have recently found use in certain applications such as in cellular phones, camcorders, and laptop computers, and even more recently, in larger power applications such as in electric vehicles and hybrid electric vehicles. It is preferred in these applications that the secondary batteries have the highest specific capacity possible but still provide safe operating conditions and good cycleability so that the high specific capacity is maintained in subsequent recharging and discharging cycles.
Although there are various constructions for secondary batteries, each construction includes a positive electrode (or cathode), a negative electrode (or anode), a separator that separates the cathode and anode, and an electrolyte in electrochemical communication with the cathode and anode. For secondary lithium batteries, lithium ions are transferred from the anode to the cathode through the electrolyte when the secondary battery is being discharged, i.e., used for its specific application. During this process, electrons are collected from the anode and pass to the cathode through an external circuit. When the secondary battery is being charged or recharged, the lithium ions are transferred from the cathode to the anode through the electrolyte.
Historically, secondary lithium batteries were produced using non-lithiated compounds having high specific capacities such as TiS2, MoS2, MnO2 and V2O5, as the cathode active materials. These cathode active materials were coupled with a lithium metal anode. When the secondary battery was discharged, lithium ions were transferred from the lithium metal anode to the cathode through the electrolyte. Unfortunately, upon cycling, the lithium metal developed dendrites that ultimately caused unsafe conditions in the battery. As a result, the production of these types of secondary batteries was stopped in the early 1990""s in favor of lithium-ion batteries.
Lithium-ion batteries typically use lithium metal oxides such as LiCoO2 and LiNiO2 as cathode active materials coupled with a carbon-based anode. In these batteries, the lithium dendrite formation on the anode is avoided thereby making the battery safer. However, the lithium, the amount of which determines the battery capacity, is totally supplied from the cathode. This limits the choice of cathode active materials because the active materials must contain removable lithium. Furthermore, the delithiated products corresponding to LiCoO2 and LiNiO2 that are formed during charging (e.g. LixCoO2 and LixNiO2 where 0.4 less than x less than 1.0) and overcharging (i.e. LixCoO2 and LixNiO2 where x less than 0.4) are not stable. In particular, these delithiated products tend to react with the electrolyte and generate heat, which raises safety concerns.
The present invention is a secondary battery having a high specific capacity and good cycleability and that operates safely. In accordance with the invention, the freshly prepared, secondary battery includes an anode that is formed of a host material capable of absorbing and desorbing lithium in an electrochemical system and lithium metal dispersed in the host material. Preferably, the lithium metal is a finely divided lithium powder and more preferably has a mean particle size of less than about 20 microns. The host material comprises one or more materials selected from the group consisting of carbonaceous materials, Si, Sn, tin oxides, composite tin alloys, transition metal oxides, lithium metal nitrides and lithium metal oxides. Preferably, the host material comprises a carbonaceous material and more preferably comprises graphite.
The freshly prepared, secondary batteries of the invention include a positive electrode including an active material, a negative electrode comprising a host material capable of absorbing and desorbing lithium in an electrochemical system and lithium metal dispersed in the host material, a separator separating the positive electrode and the negative electrode and an electrolyte in communication with the positive electrode and the negative electrode. Preferably, the cathode active material is a compound that can be lithiated at an electrochemical potential of 2.0 to 5.0 V versus lithium. For example, the cathode active material can be MnO2, V2O5 or MoS2, or a mixture thereof. The lithium metal in the anode is preferably a finely divided lithium powder and more preferably has a mean particle size of less than about 20 microns. The host material comprises one or more materials selected from the group consisting of carbonaceous materials, Si, Sn, tin oxides, composite tin alloys, transition metal oxides, lithium metal nitrides and lithium metal oxides. Preferably, the host material in the negative electrode comprises a carbonaceous material and, more preferably, comprises graphite. The amount of lithium metal present in the negative electrode is preferably no more than the maximum amount sufficient to intercalate in, alloy with, or be absorbed by the host material in the negative electrode. For example, if the host material is carbon, the amount of lithium is preferably no more than the amount needed to make LiC6.
The present invention also includes a method of preparing a freshly prepared anode for a secondary battery that includes the steps of providing a host material that is capable of absorbing and desorbing lithium in an electrochemical system, dispersing lithium metal in the host material and forming the host material and the lithium metal dispersed therein into an anode. The lithium metal and the host material is preferably mixed together with a non-aqueous liquid to produce a slurry and then applied to a current collector and dried to form the anode. Alternatively, the anode can be formed by chemical means by immersing the host material in a suspension of lithium metal in a non-aqueous liquid, and then formed into an anode.
The present invention further includes a method of operating a secondary battery. First, a freshly prepared, secondary battery is provided that includes a positive electrode including an active material, a negative electrode comprising a host material capable of absorbing and desorbing lithium in an electrochemical system and lithium metal dispersed in the host material, a separator for separating the positive electrode and the negative electrode, and an electrolyte in communication with the positive electrode and the negative electrode. In particular, the secondary battery is manufactured with lithium metal dispersed in the host material of the anode. The freshly assembled battery is in a charged state and more preferably is in a fully charged state (with all the removable lithium present in the anode of the freshly prepared battery). The freshly prepared secondary battery is initially discharged by transmitting lithium ions from the negative electrode to the positive electrode through the electrolyte. The secondary battery can then be charged or recharged by transmitting lithium ions from the positive electrode to the negative electrode through the electrolyte and then discharged again by transmitting lithium ions from the negative electrode to the positive electrode through the electrolyte. The charging and discharging steps can occur for numerous cycles while maintaining the high specific capacities of the cathode active materials and maintaining safe operating conditions.
These and other features and advantages of the present invention will become more readily apparent to those skilled in the art upon consideration of the following detailed description and accompanying drawing, which describe both the preferred and alternative embodiments of the present invention.